1. Field of the Invention
The present invention relates to a semiconductor memory device and a driving method thereof, and more particularly relates to an FBC (Floating Body Cell) memory storing information by accumulating majority carriers into a floating body of a field-effect transistor.
2. Related Art
In recent years, an FBC memory device is available as a semiconductor memory device expected as a memory substituting an IT (Transistor)-IC (Capacitor) DRAM. The FBC memory device is formed with an FET (Field Effect Transistor) having a floating body (hereinafter, also called a body) on an SOI (Silicon On Insulator) substrate. The FBC memory device stores data “1” or data “0” depending on a size of the number of majority carriers accumulated in this body. For example, in an FBC including an N-type FET, a state that the number of holes accumulated in the body is large is set as data “1” and a state that the number of those holes is small is set as data “0”. A memory cell storing the data “0” is called a “0” cell and a memory cell storing the data “1” is called a “1” cell.
In writing the data “1” into memory cells, a high-level potential is applied to word lines and bit lines and holes are accumulated in the body by impact ionization. In writing the data “0” into memory cells, a low-level potential (for example, a source potential (ground potential)) is applied to the bit lines in a state that a word line potential is at a high level. As a result, the holes are extracted from the body to a drain. In this way, in the FBC, the data “1” or the data “0” can be selectively written into plural memory cells connected to a certain selective word line.
In reading data, a sense amplifier detects a difference between driving forces of memory cell transistors due to a difference between a body potential (a signal amount) in the “0” cell and a body potential (a signal amount) in the “1” cell. However, when an operation voltage is lowered by miniaturization in the FBC, a sufficient difference between a body voltage in the “0” cell and a body voltage in the “1” cell cannot be obtained. As a result, a sufficient difference between the signal amount in the “0” cell and the signal amount in the “1” cell cannot be obtained.
That is, when miniaturization is progressed, in the conventional data writing operation, a signal amount cannot be made sufficiently large for the sense amplifier to be able to sufficiently detect the data “0” and the data “1” during the reading operation.